For cleaning NOx contained in exhaust gas discharged from diesel engines of automobiles and the like, catalytic members having honeycomb structure (honeycomb catalytic members) are currently used. These honeycomb catalytic members have such a structure that a catalytic layer 15 is carried on surface of a partition wall 4 forming a cell 3 as shown in FIG. 8. As shown in FIG. 9, in cleaning exhaust gas using this honeycomb catalytic member 60 (honeycomb structure member 11), exhaust gas is caused to flow into the cell 3 of the honeycomb catalytic member 60 from the side of one end face 2a, and to come into contact with a catalytic layer (not shown) on surface of the partition wall 4, and then to flow outside from the side of the other end face 2b (see, Patent document 1, for example).
When exhaust gas is purified using such a honeycomb catalytic member, it is necessary to promote transferring of components to be purified contained in the exhaust gas toward the catalytic layer on the partition wall surface from the exhaust gas as much as possible, and to improve the purifying efficiency. In order to improve the purifying efficiency of exhaust gas, it is necessary to decrease the hydraulic diameter of cell, and to increase the surface area of the partition wall. Concretely, an approach of increasing cell number per unit area (cell density) or the like is employed.
Here, it is known that transmissibility of components to be purified from exhaust gas to a catalytic layer on surface of partition wall increases in inverse proportion to square of hydraulic diameter of cell. Therefore, the higher the cell density, the better the transmissibility of components to be purified is. However, pressure loss also tends to increase in inverse proportion to square of hydraulic diameter of cell. Therefore, improvement in transmissibility of components to be purified is accompanied with the problem of increased pressure loss.
Thickness of a catalytic layer on surface or partition wall is typically about several tens micrometers. When the components to be purified diffuse in the catalytic layer at insufficient speed, the purifying efficiency of the honeycomb catalytic member tends to decrease. This tendency is particularly significant in a low-temperature condition. Therefore, in order to increase the purifying efficiency of exhaust gas, it is necessary to increase the diffusing speed of the components to be purified in the catalytic layer not only by increasing surface area of catalytic layer, but also by reducing the thickness of the catalytic layer. Increasing the cell density still poses the problem of increase in pressure loss although it advantageously increases surface area of catalytic layer.
In order to reduce pressure loss while increasing purifying efficiency of exhaust gas, it is necessary to increase the flow-in diameter of honeycomb catalytic member, and to decrease flow rate of circulating exhaust gas. However, when a honeycomb catalytic member is enlarged in size, it may become difficult to be mounted, for example, as an in-car honeycomb catalytic member because of limitation of mounting space. [Patent document 1] Japanese patent application laid-open JP-A 2003-33664